A new study suggests low levels of a specific biomarker in your blood is linked to the severity and duration of depression.

The collaborative study conducted by Stanford University and The Rockefeller University found people with depression had low levels of acetyl-L-carnitine, which helps the body produce energy.

Although it's naturally produced in the body, acetyl-L-carnitine is also available for purchase at drug stores and other retailers as a nutritional supplement.

In a statement, Natalie Rasgon, an author on the study and professor of psychiatry and behavioral sciences at Stanford, said the study offers "an exciting addition to our understanding of the mechanisms of depressive illness."

The study was published on July 30 in the journal Proceedings of the National Academy of Sciences.

A series of experiments involving rodents found a deficiency in the biomarker was connected to more behaviors related to depression. When the animals received acetyl-L-carnitine as a supplement, their behavior returned to normal, researchers said.

In a separate study involving humans, researchers examined patients who had been diagnosed with depression. Results showed depressed patients had much lower levels of acetyl-L-carnitine in their blood. Researchers also found patients with the lowest levels had the most severe cases of depression.

"In patients with depression, something is causing a problem in the mechanisms related to the biology of (acetyl-L-carnitine)," said Carla Nasca, a scientist at The Rockefeller University and one of the study's authors, in a statement. "And, surprisingly, the deficiency in (acetyl-L-carnitine) is even stronger in patients that don’t respond to standard antidepressants."

Rasgon said further research is required to determine whether supplementing patients with acetyl-L-carnitine could improve symptoms. She also warned about taking drug store supplements.

"We have many previous examples of how nutritional supplements widely available over the counter and unregulated by the Food and Drug Administration — for example, omega-3 fatty acids or various herbal substances — are touted as panaceas for you-name-it, and then don’t pan out," she said.